…the installation of an exhaust fan / vent for a currently un-ventilated bathroom. Steam condensation is a big problem and we’re worried about future water damage…
I have a suggestion for a workaround that might save you some bother: Our bathroom is internal to our unit without any window that could be opened. It is thus dependent on the exhaust fan. The problem was that we got a lot of steam condensation despite running the fan and having added shower dome (https://www.showerdome.com.au/). The shower dome is great for making the shower more comfortable in cold weather however by keeping in much of the steam and cutting out drafts.
The fix that worked brilliantly was to run a desiccant dehumidifier (https://ausclimate.com.au/products/cool-seasons-premium-10l-desiccant-dehumidifier) in the bathroom just during the time that we are showering. It works so well that we do not bother running the fan. We get no condensation whatsoever. The cost to run it is low since it is only needed for the duration of a shower. It does use more energy than a compressor-type dehumidifier, which would be suitable in a warmer area, but the benefit in winter is slight warming of the bathroom.